Multilayers are composed of periodic repetitions, and in the most simple case a period consists of two layers. The one layer material should consist of a so-called spacer material, while the other layer material should consist of a so-called absorber material. Spacer material has a real part of the refractive index close to 1, absorber material has a real part of the refractive index significantly different from 1. The period thickness and the thicknesses of the individual layers are chosen in dependence on the operating wavelength, so that the reflectivity is generally maximized at this operating wavelength.
Depending on the requirement of the reflective optical element in regard to the reflection profile, various configurations of the multilayer system are conceivable. Bandwidth and reflectivity, for example, can be adjusted by having more than just two materials in one period or by deviating from a constant layer thickness or even from constant thickness ratios (so-called depth-graded multilayers).
EUV lithography appliances are used in the production of semiconductor components, such as integrated circuits. Lithography appliances which are used in the extreme ultraviolet wavelength region primarily have multilayer systems of molybdenum and silicon, for example, as the optical reflective element. Although EUV lithography appliances have a vacuum or a residual gas atmosphere in their interior, it is not entirely possible to prevent hydrocarbons and/or other carbon compounds from being inside the appliance. These carbon compounds are split apart by the extreme ultraviolet radiation or by secondary electrons; resulting in the depositing of a carbon-containing contamination film on the optical elements. This contamination with carbon compounds leads to substantial reflection losses of the functional (optical surfaces, which can have a considerable influence on the economic effectiveness of the EUV lithography process. This effect is intensified in that typical EUV lithography appliances have eight or more reflective optical elements. Their transmission is proportional to the product of the reflectivities of the individual optical reflective elements.
The contamination leads not only to reflectivity losses, but also to imaging errors, which in the worst case make an imaging impossible. Thus, cleaning cycles have to be provided when operating an EUV lithography appliance or when using reflective optical elements. These significantly increase the operating costs. But the cleaning cycles not only increase the down time, but also entail the risk of worsening of the homogeneity of the layer thickness of the reflective optical elements and the risk of increasing the surface relief, which leads to further reflectivity losses.
One approach to controlling the contamination for Mo/Si multilayer mirrors is found in M. Malinowski et al., Proceedings of SPIE Vol. 4688 (2002), pages 442 to 453. A multilayer system of 40 pairs of molybdenum and silicon with pair thickness of 7 nm and a Γ=(dMo/(dMo dSi), with dMo being the thickness of the molybdenum layer and dSi the thickness of the silicon layer, of around 0.4, was provided with an additional silicon layer on the uppermost molybdenum layer. Multilayer systems with different thickness of silicon protective layer were measured, extending from 2 to 7 nm. Traditional Mo/Si multilayer systems have a silicon protective layer of 4.3 nm, which helps protect against contamination, although it very quickly becomes oxidized. The measurements revealed that there is a reflectivity plateau for a silicon protective layer of 3 nm, depending on the radiation dose. It is therefore recommended to use silicon protective layers with a thickness of 3 nm, instead of silicon protective layers with a thickness of 4.3 nm. For a longer operating time can be achieved with a silicon protective layer 3 nm in thickness, for the same tolerance in the reflectivity loss.